The utilization of repulped, latex-coated paper or paperboard as a component of a paper or paperboard pulp furnish places severe demands on the papermaking machines wet press felt conditioning system. Based upon the typical coating weights in normal use of such repulped material, the potential non-fibrous press felt contaminants can be 5 to 10 times higher than in a papermaking furnish not containing repulped, coated material. This high level of contaminating material can plug or foul the papermaking felts, severely impairing their water absorbing capabilities which, in turn, reduces machine production rates, produces paper quality defects and/or requires premature removal and replacement of the felts all leading to increased operating costs and lost production time.
Chemical analysis of used press felts which have been subjected to this type of contamination indicate the presence of significant quantities of paper coating components such as polyvinyl acetate or styrene-butadiene latex binders as well as inorganic coating pigments such as clay, calcium carbonate and titanium dioxide. In many cases, the analysis also revealed relatively small amounts of natural wood resins such as fatty esters, fatty acids, resin acids, and other typical papermaking furnish components such as sizing agents, alumina and fiber fines. Although variable in composition, this type of chemical contamination is known generally as white pitch. It is important to note, therefore, that white pitch is significantly different chemically than normally occurring wood pitch; accordingly, treatments for one will not necessarily be successful for the other.
Effective chemical conditioning of a press felt helps to reduce the rate of felt compaction, maintain maximum felt absorbency and prolong the felts useful operating life. A felt must be kept clean of filling materials, such as white pitch, which adheres to the felt fibers and accumulates in the felt structure. The filling materials not only impede the flow of water through the felt, but also create adhesion between felt fibers thus increasing the tendency for the felt structure to compact and lose absorption capacity. Prior art felt conditioners have proven to possess limited efficiency against white pitch contamination at commercially acceptable treatment dosage levels.